Field of the Invention
The present invention relates to an organic light emitting display, and more particularly, to an organic light emitting display including a device for sensing the threshold voltage of a driving TFT and a method for sensing the threshold voltage of a driving TFT in an organic light emitting display.
Discussion of the Related Art
An active-matrix organic light emitting display comprises organic light emitting diodes (OLEDs) that are self-luminous (i.e., emit light themselves). An active-matrix organic light emitting display has advantages including fast response time, high luminous efficiency, high luminance, and wide viewing angle. An OLED comprises an anode and a cathode, as well as organic compound layers HIL, HTL, EML, ETL, and EIL formed between the anode and cathode. The organic compound layers comprise a hole injection layer HIL, a hole transport layer HTL, an emission layer EML, an electron transport layer ETL, and an electron injection layer EIL. When an operating voltage is applied to the anode and the cathode, a hole passing through the hole transport layer HTL and an electron passing through the electron transport layer ETL move to the emission layer EML, thereby forming an exciton. As a result, the emission layer EML generates visible light.
In an organic light emitting diode display, pixels each comprising an organic light emitting diode are arranged in a matrix, and the luminance of the pixels is adjusted based on the grayscale of video data. Each individual pixel comprises a driving TFT (thin-film transistor) that controls the drive current flowing through the OLED. The electrical characteristic of the driving TFT, such as threshold voltage, mobility, etc., may vary from pixel to pixel because of the process condition, driving environment, etc. Such variation in the electrical characteristics of the driving TFT causes luminance differences between the pixels. As a solution to this problem, a technology that senses the characteristic parameters (threshold voltage, mobility, etc.) of the driving TFT of each pixel and corrects image data based on the sensing results is known.
In the related art, as shown in FIG. 1, a driving TFT DT is operated according to a source follower method, and then the source node voltage Vs of the driving TFT DT is detected as a sensing voltage Vsen at the time to when the gate-source voltage Vgs of the driving TFT DT reaches saturation state by an electric current flowing through the driving TFT DT, thereby sensing a change in the threshold voltage Vth of the driving TFT DT. However, a long period of time is needed for the gate-source voltage Vgs of the driving TFT DT to reach the threshold voltage Vth of the driving TFT DT. Accordingly, in the related art, it is not possible to sense a change in the threshold voltage Vth of the driving TFT DT during real-time operation.